I recall that in at least one case an F-35 used for an airshow demonstration had visible deterioration of the stealth material forward of its canopy. It seems that the low-observable systems used by the F-35 are very maintenance-intensive and that the jets used for shows may not be in combat-ready condition.

(or, if you prefer, maybe the jets used for demonstrations are intentionally damaged in visible ways so that there is a convenient excuse for the ease of detecting them on radar)

edit: I was thinking of the F-22, see comments below for a source. But similar considerations probably apply to the F-35.